Most communication systems are versatile in that they provide a variety of services, or capabilities, to their users--not just one service or capability. Even as simple a communication system as two transceivers interconnected by a wire may be thought of as providing a user connected to one of the transceivers with at least two capabilities: the capability to transmit communications over the wire, and the capability to receive communications over the wire.
Often, it is desirable to limit access by different users, or different communities of users, to different subsets of the total capabilities provided by the system, while allowing those users or user communities to share use of the system. For this purpose, the art has made use of the notion of a "class-of-service". A "class-of-service" is a specification of the communication-related capabilities that are available to a user. A number of different classes are defined, and different users are assigned to those different classes.